This invention relates to a surface acoustic transducer.
A surface acoustic wave transducer is formed of comb-shaped electrodes provided on a piezoelectric substrate, the size of the electrode being proportional to a wavelength .lambda. of a surface acoustic wave. The wavelength .lambda. is determined by the equation: .lambda.=v/f, where v is a propagation rate, and f the frequency of the surface acoustic wave transducer. Therefore, when the frequency f is high, the wavelength .lambda. is short, and in turn, the size of the electrode is small, which leads to a lowering of the yield in the manufacturing of such a transducer. It is therefore desirable to use a substrate for the surface acoustic wave transducer which has an acoustic propagation rate which is as high as possible.
Conventionally, the substrate for a surface acoustic wave transducer employs various glass substrates, such as quartz glass, with a film of zinc oxide (ZnO) as a piezoelectric element, in which the acoustic propagation rate is fairly high and in which an effective electromechanical coupling coefficient K.sup.2 (which is a basic factor for determining the transducer performance), has not been suitabley obtained. For example, in the case of using a quartz glass substrate having a ZnO film, the acoustic propagation rate v has been about 2.7 km/sec and its coefficient K.sup.2 about 3%.
On the other hand, materials having a high acoustic propagation rate include a piezoelectric material such as a film of aluminum nitride, which has a high acoustic propagation rate of 6 km/sec and is easy to process into a comb-shaped electrode, but has a defect in that the preparation of a film of aluminum nitride is difficult.